A remote antenna unit is required where physical or thermal limitations prevent placing an entire communication unit/device in an antenna unit. Such limitations may exist for example for an antenna installed on a vehicle roof, an antenna exposed to high or low temperatures, or an antenna requiring a defined and restricting mechanical size and/or shape, the latter arising from practical and/or aesthetic considerations. If the distance between the remote antenna unit and the communication unit is relatively small (e.g. less than 1 meter), the two may be connected by a coax line. However, for larger distances or when the frequency band employed is above several (e.g. 5) GHz, the attenuation is too high and coax line loss degrades performance. The performance loss is duplicated for Transmit (TX) and Receive (RX) functionality. For example, an apparently “low” attenuation of 3 dB translates into 6 dB total attenuation, because the Transmit power is 3 dB lower than a maximal power amplifier output and the Receive sensitivity is 3 dB higher than optimally achievable.
There are three known types of remote antenna units: a) without any amplification (passive); b) with fixed gain but no power control loop in the antenna unit; and c) with variable gain and power control loop in the antenna unit. Type (a) is trivial and is commonly used. Type (b) is unusable. Regarding type (c), the known art may be understood with reference to, exemplarily, U.S. Pat. No. 7,965,977 by Angell et al. Such known art is shown schematically in FIG. 1, in which an apparatus embodiment 100 comprises a remote variable gain antenna unit 101 coupled to a feeding (communication) unit 102 through a long coax line 116. As used herein, “long” with reference to coax lines refers to a length greater than about 1 meter for frequencies greater than about 5 GHz. The antenna unit is further coupled to an antenna 120.
Feeding unit 102 includes a feeding transceiver 110 for up-converting and down-converting RF signals and a baseband device 112 for modem signal processing and for controlling the transceiver mode of operation and communicating this mode to the remote antenna unit. Remote antenna unit 101 includes a power control module 104, a TX/RX control module 106 and a RF circuits module 108. Power control module 104 controls the power and gain of a power amplifier (not shown) in RF circuits module 108, while TX/RX control module 106 controls the mode of operation (i.e. Transmit or Receive) of RF circuits module 108. RF circuits module 108 further includes a low noise amplifier (LNA) (not shown) and switches (not shown) which compensate for coax line losses and facilitate the sharing of TX/RX and RF lines on coax line 116. Diplexers 114 are provided in both remote antenna unit 101 and feeding unit 102 and are used for combining the TX/RX control line signals with the RF signal on the same coax line (116). Coax line 116 carries three signals: a Transmit RF signal 140, a Receive RF signal 142, and a TX/RX control 146. The Transmit RF signal and the TX/RX controls are sent from the feeding unit to the remote antenna unit, while the Receive RF signal is sent in the opposite direction. Antenna 120 may be placed on a printed circuit board carrying the RF circuits, or connected through a very short coax line to module 108.
Power control is essential for compensating for component behavior in view of temperature changes, lifetime changes and manufacturing differences. Exemplarily, an outdoor antenna unit is exposed to higher temperature variations than an indoor antenna unit. At high frequencies, RF component properties tend to change considerably due to the factors mentioned above. An enhanced level of power control may be achieved by setting a different power per packet. However, while required with most modern communication protocols, this enhanced level cannot be supported with the configuration in FIG. 1.
The implementation of power control inside a remote variable gain antenna unit (e.g. power control module 104) is possible only when all three conditions listed next are met:
a) The transmission power is fixed—i.e. all packets are transmitted using the same power. Most modern communication systems apply dynamic transmission power control. If transmission power control is performed in the antenna unit, then the information of the present transmission power has to be provided to the remote antenna. This is not possible with a single control line (116) of the type disclosed in U.S. Pat. No. 7,965,977.
b) The amplifier output power is not high. Adjustment of transmission power at the power amplifier can be performed only when its output is relatively low. It cannot be performed when it is high, since then it may result in unstable or non-linear operation.
c) The antenna unit size requirements are not restricting—i.e. room exists for implementing the power control function in the remote antenna unit.
When at least one of these three conditions is not met, a new scheme is needed. Some applications, such as vehicle-to-vehicle communications, fail to satisfy all three conditions. There is therefore a need for and it would be advantageous to have methods and apparatuss for efficient external control of the Transmit power of a remote fixed gain antenna unit with power feedback, in particular (but not limited to) for efficient external control performed over a single coax line in a vehicular environment.